In the oil-exploring industry, gas-liquid mixed fluids comprising liquid phase and gas phase are usually explored from oil wells, and the fluids are customarily called as “wet gas” in the art. Said gas phase includes, for example, air, oil field gas or any gases which are non-condensable at room temperature, wherein the oil field gas is generally selected from relatively light alkanes, such as methane, ethane, propane, butane and the like. Said liquid phase includes crude oil per se, and water and other liquid additives which are injected to oil wells during the exploration. The liquid volume flow rate and the gas volume flow rate of the gas-liquid mixed fluids explored in oil wells, which are real-time and accurately measured, are necessary basic data for the production management and production optimization.
Currently, there are some methods which can achieve the on-line measurement of gas volume flow rate and liquid volume flow rate of a gas-liquid mixed fluid.
In conventional methods, a gas-liquid mixed fluid is separated into a gas phase and a liquid phase via a separator, and then the volume flow rate of the two phases can be respectively metered. However, because the separator and relevant instillations affiliated thereto weigh to be decadal tons, occupy a space having an area of hundreds square meters, and have many controlling steps, maintenances and managements for the separator are complex, which is disadvantageous to automation of the management to production procedure, in particular, disadvantageous to the use in oil fields in desert or ocean.
In another kind of methods, gas and liquid phases are not separated, while the total volume flow rate Q and gas volume fraction GVF of a gas-liquid mixed fluid are measured so as to measure gas volume flow rate and liquid volume flow. The gas volume fraction GVF refers to the percentage of the gas flow rate in the total flow rate of the gas-liquid mixed fluid, and it also can refer to the ratio of the area occupied by the gas phase to the whole cross-section area at a certain cross-section. The calculation method under theoretical circumstance comprises the following equations: the gas volume flow rate Qg=Q×GVF, and the liquid volume flow rate Ql=Q×(1−GVF). However, such measuring methods should be based on the proviso that the gas and liquid phases should have the same speed at the cross-section.
However, in fact, because gas phase and liquid phase are different from each other markedly in the density, viscosity and other properties, the above proviso is not tenable in actual pipeline. In actual pipeline, because gas phase has a relatively low density, the velocity of gas phase is often higher than that of liquid phase, which results in that the above gas and liquid volume flow rate measured under theoretical circumstance will involve some errors. Hence, there is a need for an improved method to correct the above theoretical circumstance. In the art, the difference between the gas velocity and the liquid velocity of the gas-liquid mixed fluid in a same pipeline can be referred to “gas-liquid slip”, and the ratio of the gas velocity to the liquid velocity of the gas-liquid mixed fluid in a same pipeline can be referred to “gas-liquid slip factor”. Detailed introductions to the gas-liquid slip and the gas-liquid slip factor will be made in the following text.
In current measuring techniques for a gas-liquid mixed fluid, there are two methods to correct the errors caused by the gas-liquid slip:
One method relates to the iterative calculation based on the Lockhart-Martinelli parameters of gas phase and liquid phase, principally used in the correction for overly high flow rate in the measurement of the wet gas containing a very high percentage of gas. Typical representation is the ISO wet gas model (for example, please see the publication ISO/TR 11583:2012, with the English title “measurement of wet gas flow by means of pressure differential devices inserted in circular cross-section conduits”). Such method uses a single phase meter to measure wet gas as a single phase, and multiphase meters will not be used. Hence, liquid flow rate value or an approximating value thereof should be acquired beforehand via other routes, and thus the correction is the correction directed to the gas flow. Such method has three defects: 1. there are no explicit dynamic mechanism; 2. only the gas flow rate is corrected, and no corrections to the metering of the liquid flow rate are made; and 3. the applicable scope is narrow because the method is limited to wet gas containing a very high percentage of gas.
Another method relates to the correction to the gas-liquid slip of the two phase flow based on an empirical model, for example, experimental data fitting mode, or sign treatment model. Such method has two defects: the dependence of the empirical model on experimental data and measuring conditions is strong, and thus the method cannot achieve the balance of the universality and the precision; in addition, due to the complexity of the flow state of a multiphase fluid, it is very difficult for the empirical model to make selections to the modeling parameters and make more reasonable hypothesis to the mathematical relation of the parameters. In the signal processing, multiphase fluids are treated completely as a black box, and thus the calculation precision and scope have significant artificial features and uncertainty.
Hence, there is a need in the art for a method which can simply and precisely measure gas volume flow rate and liquid volume flow rate of a gas-liquid mixed fluid, and which can correct the measurement errors caused by the gas-liquid slip.
The above drawings are provided merely for illustrations, and they are not intended to limit the present invention in any manner. The protection scope of the invention is determined only by the claims.